Modulation signals for a satellite navigation system

ABSTRACT

A method of generating a subcarrier modulation signal for modulating a further signal, the method involving multiplexing or selectively combining portions of first and second subcarriers to produce the subcarrier modulation signal.

FIELD OF THE INVENTION

The invention relates to modulation signals, systems and methods suchas, for example, navigation and positioning signals, systems andmethods.

BACKGROUND TO THE INVENTION

Satellite Positioning Systems (SPS) rely on the passive measurement ofranging signals broadcast by a number of satellites, or ground-based orairborne equivalents, in a specific constellation or group ofconstellations. An on-board clock is used to generate a regular andusually continual series of events, often known as ‘epochs’, whose timeof occurrence is coded into a random or pseudo-random code (known as aspreading code). As a consequence of the pseudo-random or randomfeatures of the time epoch encoding sequence, the spectrum of the outputsignal is spread over a frequency range determined by a number offactors including the rate of change of the spreading code elements andthe waveform used for the spreading signal. Typically, the spreadingwaveform is rectangular and has a sinc function power spectrum.

The ranging signals are modulated onto a carrier signal for transmissionto passive receivers. Applications are known that cover land, airborne,marine and space use. Typically, binary phase shift keying is employedto modulate the carrier signal, which, itself, has a constant magnitude.Usually, at least two such signals are modulated onto the same carrierin phase quadrature. The resulting carrier signal retains its constantenvelope but has four phase states depending upon the two independentinput signals. However, it will be appreciated that two modulatingsignals do not need to have the same carrier magnitude. It is possiblefor a constant carrier magnitude of the combined signal to be maintainedby appropriate selection of corresponding phases other than π/2 radians.

An example of such a satellite positioning system is the GlobalPositioning System (GPS). Generally, the GPS operates using a number offrequencies such as, for example, L1, L2 and L5, which are centred at1575.42 MHz, 1227.6 MHz and 1176.45 MHz respectively. Each of thesesignals is modulated by respective spreading signals. As will beappreciated by those skilled in the art, a Coarse Acquisition (CA) codesignal emitted by the GPS Satellite Navigation System is broadcast onthe L1 frequency of 1575.42M with a spreading code rate (chip rate) of1.023 MHz. The CA has a rectangular spreading waveform and iscategorised as BPSK-R1. The GPS signal structure is such that the signalbroadcast by the satellites on the L1 frequency has a second componentin phase quadrature, which is known as the precision code (P(Y) code)and made available to authorised users only. The P(Y) signal is BPSKmodulated with a spreading code at 10.23 MHz with a magnitude which is 3dB lower in signal power than the CA code transmission. Consequently,the Q component has a magnitude which is 0.7071 (−3 dB) of the magnitudeof the I component. It will be appreciated by those skilled in the artthat the phase angles of these states of these signals are ±35.265° inrelation to the ±I axis (phase of the CA code signal as specified in ICDGPS 200C). One skilled in the art also appreciates that the P code is afunction of or is encrypted by the Y code. The Y code is used to encryptthe P code. One skilled in the art appreciates that the L1 signal,containing both I & Q components, and the L2 signal can be represented,for a given satellite, i, asS _(L1i)(t)=A _(p) p _(i)(t)d _(i)(t)cos(ω₁ t)+A _(C) c _(i)(t)d_(i)(t)sin(ω₁ t), andS _(L2i)(t)=B _(p) p _(i)(t)d _(i)(t)cos(ω₂ t)whereA_(P) and A_(C) are the amplitudes of the P and CA codes, typicallyA_(P)=2A_(C);B_(p) is the amplitude of the L2 signal;ω₁ and ω₂ are the L1 and L2 carrier frequencies;p_(i)(t) represents the P(Y) ranging code and is a pseudo-randomsequence with a chip rate of 10.23 Mcbps. The P code has a period ofexactly 1 week, taking values of +1 and −1;c_(i)(t) represents the CA ranging code and is a 1023 chip Gold code,taking values of +1 and −1;d_(i)(t) represents the data message, taking values of +1 and −1.

A satellite constellation typically comprises 24 or more satellitesoften in similar or similarly shaped orbits but in a number of orbitalplanes. The transmissions from each satellite are on the same nominalcarrier frequency in the case of code division access satellites (suchas GPS) or on nearby related frequencies such as GLONASS. The satellitestransmit different signals to enable each one to be separately selectedeven though several satellites are simultaneously visible.

The signals from each satellite, in a CDMA system like GPS, aredistinguished from each one another by means of the different spreadingcodes and/or differences in the spreading code rates, that is, thep_(i)(t) and c_(i)(t) sequences. Nevertheless, as will be appreciatedfrom the power spectrum 100 shown in FIG. 1 there remains significantscope for interference between the signals transmitted by thesatellites. FIG. 1 shows power spectra 100 for the CA and P(Y) codes.The power spectrum 102 for the CA code has maximum power at the carrierfrequency L1 and zeros at multiples of the fundamental frequency, 1.023MHz, of the CA code. For example, it can be appreciated that zeros occureither side of the carrier frequency at ±1.023MHz, ±2.046MHz etc.Similarly, the power spectrum 104 for the P(Y) code has a maximumamplitude centred on the L1 and L2 frequencies, with zeros occurring atmultiples of ±10.23MHz as is expected with a sine function waveform.

It is known to further modulate the ranging codes using a sub-carrier,that is, a further signal is convolved with the P codes and/or CA codesto create Binary Offset Carrier (BOC) modulation as is known within theart see, for example, J. W. Betz, “Binary Offset Carrier Modulation forRadionavigation”, Navigation, Vol. 48, pp 227-246, Winter 2001-2002.Standard BOC modulation 200 is illustrated in FIG. 2. FIG. 2 illustratesthe combination of a portion of a CA code 202 with a subcarrier signalto produce the BOC signal 204 used to modulate a carrier such as, forexample, L1. It can be appreciated that the BOC signal is a rectangularsquare wave and can be represented as, for example,c_(i)(t)*sign(sin(2πf_(s)t)), where f_(s) is the frequency of thesubcarrier. One skilled in the art understands that BOC(f_(s), f_(c))denotes Binary Offset Carrier modulation with a subcarrier frequency off_(s) and a code rate (or chipping rate) of f_(c). Using binary offsetcarriers results in the following signal descriptions of the signalsemitted from the satellite:S _(L1i)(t)=A _(m) sc _(im)(t)m _(i)(t)d _(i)(t)cos(ω₁ t)+A _(C) sc_(ig)(t)g _(i)(t)d _(i)(t)sin(ω₁ t)=I _(SL1i)(t)+Q _(SL1i)(t), andS _(L2i)(t)=B _(m) sc _(im)(t)m _(i)(t)d _(i)(t)cos(ω₂ t)whereA_(m), A_(c) and B_(m) are amplitudes;m_(i)(t) is an m-code BOC(10,5) signal;g_(i)(t) is a Galileo open service range code;sc_(im)(t) represents the sub-carrier signal for m_(i)(t);sc_(ig)(t) represents a subcarrier signal for c_(i)(t);ω₁ and ω₂ are the L1 and L2 carrier frequencies;

FIG. 2 also illustrates power spectra for a BPSK-R1 code and pair of BOCsignals, that is, BOC(2,1) and BOC(10,5). The first spectrum 202corresponds to BPSK-R1 code. The second power spectrum 204. correspondsto the BOC(2,1) code and the third power spectrum 206 corresponds to theBOC(10,5) code. It can be appreciated that the side lobes 208 of theBOC(2,1) signal have a relatively large magnitude. Similarly, theillustrated side lobe 210 of the BOC(10,5) signal has a relatively largemagnitude. One skilled in the art appreciates that the energy in theside lobes are a source of interference.

It is an object of embodiments of the present invention to at leastmitigate the problems of the prior art.

SUMMARY OF INVENTION

Accordingly, a first aspect of embodiments of the present inventionprovides an m-level modulation signal comprising m signal amplitudes,where m>2, for modulating a first signal.

A second aspect of embodiments of the present invention provides amethod of generating a transmission signal comprising a carrier signal,the method comprising the step of combining a plurality of subcarriermodulation signals with the carrier signal.

A third aspect of embodiments of the present invention provides aranging system comprising means for generating a ranging code; means forgenerating a signal or implementing a method as claimed in any precedingclaim; means for transmitting the signal.

A fourth aspect of embodiments of the present invention provides asystem comprising memory for storing at least one of a plurality ofselectable phase states and selectable amplitude states; the memorybeing responsive to at least one of a ranging code signal, a systemclock signal and a subcarrier signal to produce a carrier signal bearingat least one of phase and amplitude modulation to produce a transmissionsignal.

Advantageously, embodiments of the present invention providesignificantly more control over the shape of the power spectra ofsignals, that is, the distribution of energy within those signals.

Other aspects of the present invention are described and defined in theclaims.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described, by way ofexample, only with reference to the accompanying drawings in which:

FIG. 1 shows a power spectrum of a pair of ranging code;

FIG. 2 illustrates power spectra of a ranging code (BPSK-R1) andBOC(10,5) signals;

FIG. 3 illustrates a multi-level sub-carrier;

FIG. 4 illustrates the phase states for at least a pair of multilevelsubcarriers according to embodiments of the present invention;

FIG. 5 depicts a power spectrum of a prior art subcarrier and asubcarrier according to embodiments of the present invention;

FIG. 6 illustrates phase states for a subcarrier according toembodiments of the present invention;

FIG. 7 illustrates in phase and quadrature phase subcarriers accordingto embodiments of the present invention;

FIG. 8 illustrates phase states of a subcarrier according to anembodiment of the present invention;

FIG. 9 shows subcarriers according to embodiments of the presentinvention;

FIG. 10 depicts power spectra of subcarriers according to embodiments ofthe present invention;

FIG. 11 illustrates subcarriers according to embodiments of the presentinvention;

FIG. 12 shows an alternative subcarrier waveform according to anembodiment of the present invention;

FIG. 13 illustrates a further alternative waveform according toembodiments of the present invention;

FIG. 14 illustrates, schematically, a transmitter using subcarriersaccording to embodiments of the present invention; and

FIG. 15 illustrates a further embodiment of a transmitter according toan embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 3, there is shown a first embodiment of a subcarrier300. It can be appreciated that the sub-carrier is a 5-levelapproximation of a sinusoidal signal 302. It can be appreciated that thesignal levels are (+1, +1/√2, 0, −1/√2, −1). Furthermore, it will beappreciated that the levels are the projections onto the x or I axis ofa rotating vector at angles of π/4 radians having a unit magnitude. Itwill be further appreciated that, given the in-phase andquadrature-phase components of, for example, S_(L1i), that is,A_(m)sc_(im)(t)m_(i)(t)d_(i)(t)cos(ω_(i)t)=I_(SL1i)(t) andA_(C)sc_(ig)(t)g_(i)(t)d_(i)(t)sin(ω₁t)=Q_(SL1i)(t), the magnitude ofthe signal will be such that it is constant since the projection of thequadrature phase component onto the y or Q axis will also take thevalues (+1, +1/√2, 0, −1/√2, −1).

It will be appreciated that there are preferably restrictions on thecombinations of signals, at least one of which is that a constantmodulus signal should be maintained. The constraints are (1) that “+1”or “−1” on the in-phase component can only occur in conjunction with “0”on the quadrature phase component and visa versa and (2) “±1/√2” canonly occur on both phases simultaneously. The magnitudes of the in-phaseand quadrature phase components of the spreading signals, sc_(ig)(t) orsc_(im)(t) can be plotted on an Argand diagram 400 such as is shown inFIG. 4. The waveforms for the I and Q components are thus built from thefollowing signal element sequences:

-   -   I phase—(+1/√2, +1, +1/√2, 0) representing a +1 signal    -   I phase—(−1/√2, −1, −1/√2, 0) representing a −1 signal    -   Q phase—(+1/√2, 0, −1/√2, −1) representing a +1 signal    -   Q phase—(−1/√2, 0, +1/√2, +1) representing a −1 signal.

Any combination of I or Q signal sequences can be chosen from the aboveset within the constraint of a constant magnitude carrier signal,computed as (I²+Q²)^(1/2). It will be clear to those skilled in the art,that there are many other equivalent sets of sequences which may bechosen from the set of 5-levels satisfying the criterion of a constantcarrier envelope. It can be appreciated that the magnitudes of thesubcarriers on the I and Q channels can be thought of as being analogousto the states of an 8-PSK signal. Therefore, such a pair of 5-levelsubcarrier carrier signals can be thought of as 8 phase subcarriersignals.

FIG. 5 illustrates the effect of using a stepped or m-level, m>2,subcarrier waveform. Referring to FIG. 5 there is shown a pair 500 ofpower spectra. The first power spectrum 502, illustrated using thedotted line, represents the spectrum of a BOC(2,2) subcarrier. It can beappreciated that the energy of the subcarrier is contained withinprogressively reducing side lobes 504, 506, 508 and 510. The secondpower spectrum 512 represents the power spectrum of a BOC(2,2) signalthat used 8 phase subcarrier signals, that is, 8 phase amplitudes,represented by BOC8(2,2). More generally, BOCm(f_(s), f_(c)) representsan m-phase subcarrier signal of having a frequency of f_(s) and achipping rate of f_(c). It can be appreciated that the spectrum 512 ofthe BOC8(2,2) signal has a number of side lobes 514, 516, 518, 520, 522and 524. Of those side lobes, it can be seen that the 1^(st) to 4^(th)side lobes are significantly reduced, that is, they comprise significantless energy, as compared to the side lobes of the BOC(2,2) signalspanning the same frequencies. The significant reduction in the 1^(st)to 4^(th) side lobes can be beneficial in situations in which oneskilled in the art wishes to use the frequency spectrum spanned by theside lobes for other transmissions.

It will be appreciated by those skilled in the art the BOC8(2,2) hassignificantly improved interference properties as determined usingSpectral Separation Coefficients (SSC) and self-SSCs as is wellunderstood by those skilled in the art, that is, the spectral couplingbetween a reference signal and BOC(2,2) is greater than the spectralcoupling between a reference signal and BOC8(2,2). For example, aBOC8(2,2) signal exhibits a 10-12 dB improvement in spectral isolationas compared to a conventional BOC(2,2) signal. Further information onthe relationship between SSC and signals according to embodiments, ofthe present invention can be found in, for example, Pratt & Owen; BOCModulation Waveforms, IoN Proceedings, GPS 2003 Conference, Portland,September 2003, which is incorporated herein by reference for allpurposes and filed herewith as in the appendix.

Furthermore, embodiments of the present invention utilise the magnitudeand duration of the subcarrier to influence, that is, control the energyin harmonics of the resulting modulating waveform. For example,referring still to FIG. 5, it can be appreciated that additionalspectral nulls appear in the BOC8(2,2) spectrum at substantially 6MHzand 10MHz offset from the carrier whereas there are no such nulls in theconventional BOC(2,2) signal. The location of the nulls is influenced byat least one of the magnitude and duration of the steps in themultilevel subcarrier. More specifically, the nulls can be steered todesired locations by changing either of these two elements, that is, theposition of the nulls is influenced by these two elements. Appendix Acontains an indication of the relationship between the spectra ofsignals according to embodiments of the present invention and themagnitude and duration of the steps.

Referring to FIG. 6, there is shown subcarrier states or amplitudes forI and Q signals for a further BOC8 signal, that is, a binary offsetcarrier having eight states. It can be appreciated that the eight statescan be represented by, or correspond to, subcarrier amplitudes chosenfrom the set (−√{square root over (3)}/2, −1/2, +1/2, +√{square rootover (3)}/2) i.e. four states or signal amplitudes rather than the fivestates or signal amplitudes described above. Therefore, the I and Qcomponents, constructed from the following signal elements such that√{square root over ((cos²

+sin²

))}=1, that is, ±√{square root over (3)}/2 can only occur in conjunctionwith ±1/2, are as follows:

-   -   I phase—(+1/2, +√3/2, +√3/2, +1/2) representing a +1 chip of a        ranging code signal    -   I phase—(−1/2, −√3/2, −√3/2, −1/2) representing a −1 chip of a        ranging code signal    -   Q phase—(+√3/2, +1/2, −1/2, −√3/2) representing a +1 chip of a        ranging code signal    -   Q phase—(−√3/2, −1/2, +1/2, +√3/2,) representing a −1 chip of a        ranging code signal.

It will be appreciated that the states 1 to 8 shown in FIG. 6 are notequidistantly disposed circumferentially. The transitions between states2&3, 4&5, 6&7, 8&1 are larger in angular step than the transitionsbetween states 1&2, 3&4, 5&6, 7&8. It will be appreciated that whenthese states are translated into subcarrier amplitudes, the duration ofa given amplitude will depend on the duration or dwell time of acorresponding state, that is, the durations for which the subcarrierremains in any given state may no longer be equal unlike the states ofFIG. 4 above. The dwell times are a matter of design choice such as, forexample, to minimise the mean square difference between a steppedwaveform and a sinusoid. FIG. 7 a illustrates the subcarriers 700 and702 corresponding to the states shown in FIG. 6. It can be appreciatedthat the durations of within each state the subcarriers 700 and 702 areequal. The Q channel subcarrier magnitudes will follow substantially thesame pattern as described above but phase shifted by π/2 radians. Thesubcarrier 702 for the Q channel is shown in dotted form in FIG. 7. Itwill be appreciated that such subcarriers provide a constant envelopemagnitude since (I²+Q²)^(1/2)=1 for all amplitude combinations. However,referring to FIG. 7 b, there is shown a pair of subcarriers 704 and 706in which the durations at each state are unequal. It will be appreciatedthat not all amplitude combinations satisfy (I²+Q²)^(1/2)=1. Therefore,the transmitted signal will not have a constant envelope.

It will be appreciated by those skilled in the art that a stepped halfcycle of the subcarrier corresponds to one chip. However, otherembodiments can be realised in which other multiples of half cyclescorrespond to a chip. For example, embodiments can be realised in whichtwo half cycles of a subcarrier correspond to a chip. In suchembodiments the signals for the I and Q channels would be

I phase—(+1/2, +√3/2, +√3/2, +1/2, −1/2, −√3/2, −√3/2, −1/2)representing a +1 signal

I phase—(−1/2, −√3/2, −√3/2, −1/2, +1/2, +√3/2, +√3/2, +1/2)representing a −1 signal

Q phase—(+√3/2, +1/2, −1/2, −√3/2, −√3/2, −1/2, +1/2, +√3/2)representing a +1 signal

Q phase—(−√3/2, −1/2, +1/2, +√3/2, +√3/2, +1/2, −1/2, −√3/2)representing a −1 signal.

Similarly, embodiments realised using three half cycles per chip wouldproduce

I phase—(+1/2, +√3/2, +√3/2, +1/2, −1/2, −√3/2, −√3/2, −1/2, +1/2,+√3/2, +√3/2, +1/2) representing a +1 signal

I phase—(−1/2, −√3/2, −√3/2, −1/2, +1/2, +√3/2, +√3/2, +1/2, −1/2,−√3/2, −√3/2, −1/2) representing a −1 signal

Q phase—(+√3/2, +1/2, −1/2, −√3/2, −√3/2, −1/2, +1/2, +√3/2, +√3/2,+1/2, −1/2, −√3/2) representing a +1 signal

Q phase—(−√3/2, −1/2, +1/2, +√3/2, +√3/2, +1/2, −1/2, −√3/2, −√3/2,−1/2, +1/2, +√3/2) representing a −1 signal.

One skilled in the art will appreciate that the above can be extended ton half cycles of a subcarrier per ranging code chip.

It will be appreciated that other phases can be used to describe thesubcarriers. For example, phase and amplitude components of 16-PSK canbe used to create BOC16 subcarriers having 9 levels, assuming that thefirst state is at (+1,0). Using m-PSK phase states can be used toproduce (m+2)/2 level subcarrier signals. Therefore, setting m=2 givesthe conventional BPSK and a two-level subcarrier. Setting m=4 provides a3 level subcarrier, that is, BOC4 modulation, setting m=8 produces a 5level subcarrier, that is, BOC8 modulation, setting m=16 produces a 9level subcarrier, which corresponds to BOC16 modulation.

It will be appreciated that several further variations in the assignmentof code and data states to the phase locations can be realised. Forexample, rotation of the states shown in FIG. 4 by 22.5° leads to areassignment of angles associated with the states from the angles (0°,45°, 90°, 135°, 180°, 225°, 270°, 315°) to the angles (22.5°, 67.5°,112.5°, 157.5°, 202.5°, 247.5°, 292.5°, 337.5°). Again, it will beappreciated that this does not cause a change in the modulus of thespectrum and, again, the required number of amplitude levels reducesfrom 5 to 4, that is, m-PSK can be used to realise [(m+2)/2−1]amplitudes according to appropriate rotation and alignment of the phasestates. The resulting waveforms for the I and Q components are built, inthis case, from the following signal element sequences:

-   -   I phase—(+cos(67.5°), +cos(22.5°), +cos(22.5°), +cos(67.5°))        representing a +1 signal    -   I phase—(−cos(67.5°), −cos(22.5°), −cos(22.5°), −cos(67.5°))        representing a −1 signal    -   Q phase—(+sin(67.5°), +sin(22.5°), −sin(22.5°), −sin(67.5°))        representing a +1 signal    -   Q phase—(−sin(67.5°), −sin(22.5°), +sin(22.5°), +sin(67.5°))        representing a −1 signal.

It should be noted that the I and Q signal element sequences for thecases described above are orthogonal over the duration of one spreadingpulse (chip). Clearly, other rotations are possible and will yieldorthogonal signal element sets.

An alternative way of representing the above is via a state table.Assume that an embodiment of a BOC8 modulation has been realised withequidistant states and the first state having a phase angle of π/8radians (22.5°) as shown in FIG. 8, which correspond to the abovevalues. The sequence of phase states required for each I and Q rangingcode signal components, assuming that the ranging codes transitionsubstantially simultaneously and a desire to maintain a substantiallyconstant output envelope, that is, the states for the subcarriers wouldbe given by TABLE 1 Sequence of States for BOC8(x, x) I & Q signalelements I Q t1 t2 t3 t4 +1 +1 2 1 8 7 −1 +1 3 4 5 6 +1 −1 7 8 1 2 −1 −16 5 4 3

It will be appreciated that the subcarrier corresponding to the phasestates in Table 1 comprises a half cycle per ranging code chip.Furthermore, the sense of the phasor is clockwise when I and Q are equaland anticlockwise otherwise. It will be apparent that the signal elementsequences or state sequences are sections (specifically half cyclesections in the aspect of the invention disclosed above) of a sampled orquantised sinusoid. The concept can, therefore be extended to include amultiplicity of such samples. Those variants, which appear to be useful,include the cases with samples from a finite number of half cycles, thatis, rather than, for example, an I channel value of +1 being representedby the states of 2, 1, 8 and 7, it can be represented using some othernumber of states such as, for example, 2, 1, 8, 7, 6, 5, 4, 3, 2, 1, 8,7 i.e. by three half cycles of the sample or quantised sinusoid. Table 2illustrates the phase states for such an embodiment and is based on thephase state diagram of FIG. 4 for samples but using three half cycles(or an arbitrary number of half cycles) of the sinusoid waveform. Thesinusoid or portion or multiple of half cycles thereof is known as the‘basis waveform’. One skilled in the art realises that other basiswaveforms can be used such as, for example, a triangular waveform or aset of mutually orthogonal waveforms. TABLE 2 Sequence of States for8-PSK I & Q Signal Elements with 1½ cycles of sub-carrier per chipmodulation I Q t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 T11 t12 +1 +1 2 1 8 7 6 54 3 2 1 8 7 −1 +1 3 4 5 6 7 8 1 2 3 4 5 6 +1 −1 7 8 1 2 3 4 5 6 7 8 1 2−1 −1 6 5 4 3 2 1 8 7 6 5 4 3

TABLE 3 Sequence of States for 8-PSK I & Q Signal Elements with two halfcycles of sub-carrier per chip modulation. I Q t1 t2 t3 t4 t5 t6 t7 t8+1 +1 2 1 8 7 6 5 4 3 −1 +1 3 4 5 6 7 8 1 2 +1 −1 7 8 1 2 3 4 5 6 −1 −16 5 4 3 2 1 8 7

One skilled in the art will appreciated that it is assumed in Tables 1to 3, that the I and Q chip transitions happen substantiallysimultaneously and, furthermore, that the I and Q subcarriers take theform of sine and cosine waveforms respectively. However, embodiments canbe realised in which the ranging code chip transitions do not occursubstantially simultaneously. Furthermore, in circumstances in which theranging code chip transitions do not occur substantially simultaneously,the subcarriers corresponding to the I and Q ranging code chips can bearranged to take the form of a pair of quantised sine waves.

It will seen that there are 4 time samples for each 1/2 cycle of thewaveform. The stepped sinusoidal waveform may be viewed as sub-carriermodulation of the basic spreading waveform. The number of time samplesand independent information bearing channels is related to the number ofphase states, which the carrier signal has in its representation.Although the examples above have used phase states that are ‘powers of2’, embodiments can be realised in which some other number is used. Forexample, a 6-PSK carrier signal can be used to carry 2 independentinformation bearing binary channels. In this case only 3 signal elementsamples are required per transmitted code chip.

One skilled in the art appreciates that the replacement of the steppedsinusoid with a rectangular wave with duration of each element equal toa 1/2 cycle of the sinusoid is well known within the art. As indicatedabove, it is known as ‘Binary Offset Carrier’ modulation. There areusually 2 further attributes associated with the BOC description, whichrelate to the frequency of the code chipping rate and to the frequencyof the offset sub-carrier. BOC(2,2) consequently is interpreted as awaveform with a 2.046MHz chipping rate and a 2.046MHz offsetsub-carrier. This arrangement has exactly two 1/2 cycles of thesub-carrier signal for each code element (chip).

A further aspect of embodiments of the present invention relates tousing a set of subcarriers to modulate ranging codes, with at least oneor more, or all, of the subcarriers being multilevel waveforms. Oneskilled in the art may think of such embodiments as modulation of thesubcarrier signal by a further subcarrier signal. The resulting signaltransmitted by an ith satellite or system having a carrier frequency ofω_(i), for an additional subcarrier, would have the form:S _(i)(t)=A _(m) sc _(jm)(t)sc _(im)(t)m _(i)(t)d _(i)(t)cos(ω_(i) t)+A_(C) sc _(jg)(t)sc _(ig)(t)g _(i)(t)d _(i)(t)sin(ω_(i) t)=I _(Si)(t)+Q_(Si)(t)wheresc_(im)(t) and sc_(jm)(t) represent first and second subcarrier signalsrespectively first ranging codes such as, for example, M-codes; andsc_(ig)(t) and sc_(jg)(t) represent first and second subcarrier signalssecond ranging codes such as, for example, Gold codes. In general, for nsubcarriers, the signal would have the form:

It should be noted that embodiments can be realised in which sc_(im)(t)and sc_(ig)(t) are the same or different. Similarly, embodiments can berealised in which sc_(jm)(t) and sc_(ig)(t) are the same of different.$\begin{matrix}{{S_{i}(t)} = {{A_{m}{\prod\limits_{j = 1}^{n}\quad{{{sc}_{ijm}(t)}{m_{i}(t)}{d_{i}(t)}{\cos\left( {\omega_{i}t} \right)}}}} +}} \\{A_{C}{\prod\limits_{j = 1}^{l}\quad{{{sc}_{ijg}(t)}{g_{i}(t)}{d_{i}(t)}{\sin\left( {\omega_{1}t} \right)}}}} \\{{= {{I_{Si}(t)} + {Q_{Si}(t)}}},}\end{matrix}$where$\prod\limits_{j = 1}^{n}\quad{{{sc}_{ijm}(t)}\quad{and}\quad{\prod\limits_{j = 1}^{l}\quad{{sc}_{ljg}(t)}}}$represents the product of the subcarriers for the first and secondranging codes such as, for example, the m and Gold codes.

Although it is possible to use more than one subcarrier, practicalembodiments will typically use 2 subcarriers. Modulation using a pair ofsubcarriers is known as Double Binary Offset Carrier (DBOC) modulation.Modulation using three subcarriers is known as Triple Binary OffsetCarrier (TBOC) modulation and so on such that modulation using a n-tupleof subcarriers is known as N-tuple Binary Offset Carrier (NBOC). Asmentioned above, one or more than one of the subcarriers may be stepped,that is, having magnitudes related to respective phase states.

As examples of this aspect of the invention, FIG. 9 illustrates a pairof waveforms 900. In FIG. 9, as an illustration of the NBOC invention,the subcarrier basis waveforms are assumed to be binary and only asingle subcarrier waveform 902 is shown. The time duration in FIG. 9 is512 samples and exactly matches the duration of one code elementduration (chip). The first subcarrier 902 contains 4 half cycles of asubcarrier per ranging code chip, as illustrated by the dashed waveform.If this was the only sub-carrier component, the modulation would be aBOC(2x,x) type, where x is the frequency of the code rate (chippingrate). However, it can be appreciated that a second sub-carrier (notshown) having 16 half cycles per 512 samples has been used to producethe modulated waveform 904 to be combined with the carrier of thesatellite signal. The modulated waveform is shown by the solid curve. Asa result of modulation (multiplication) of the two subcarriers, theresulting waveform 904 has phase reversals for the second subcarrier 904whenever there is a sign reversal in the first subcarrier 902. This isclearly evident in FIG. 9 at points 906, 908 and 910, where thetransitions of the second subcarrier (not shown) would have beenopposite. The resulting modulation is denoted Double BOC, or DBOC. Inthe case of FIG. 9, the modulation is DBOC(8x,(2x,x)), that is, thereare 8 half cycles of the second subcarrier per chip of the ranging code(not shown). The main energy is concentrated around frequencies ±8x fromthe carrier signal, with a BOC like double humped spectrum.

Referring to FIG. 10, there is shown a pair of power spectra 1000. Afirst power spectrum 1002 relates to a DBOC8(16,(2,2)) signal. It willbe appreciated that at least one of the first and second subcarriersused to create the DBOC8(16,(2,2)) signal comprised amplitudes derivedfrom 8 corresponding phase states. In the specific embodiment shown, thefirst subcarrier was the multi-level signal. It will be appreciated thatthe nomenclature for representing DBOC modulation or subcarriers isDBOCa(b,c(d,e)), where a and c represent the number of phase states,that is, amplitudes, of the subcarriers having frequencies b and drespectively. The second spectrum 1004 relates to a BOC8(2,2) signal.The spectra shown have been made using a previous aspect of theinvention, that is the use of multilevel subcarriers or subcarriershaving more than two phase states, in combination with the Double BOCconcept. The waveforms for I & Q modulations for the spectrum of FIG. 10are shown in FIG. 11. Referring to FIG. 11 there is shown a pairs 1100of waveforms. The first pair of waveforms 1102, representing the Ichannel of the spreading waveform, comprises a stepped or multi-levelBOC(2,2) signal 1104, represented by the solid line, and a 16Mhzsubcarrier modulated BOC(2,2) signal 1106, represented by the dashedline. It will be appreciated that the 16 M subcarrier modulated BOC(2,2)signal has been produced by multiplying the BOC8(2,2), that is, steppedBOC(2,2) signal, by a 16 MHz rectangular waveform (not shown) havingamplitudes of ±1. The second waveform 1108, representing the Q channel,comprises a quadrature BOC(2,2) signal 1110 together with a 16MHzsubcarrier modulated BOC(2,2) signal 1112. It can be appreciated thatthe first subcarrier 1104 or 1110 is a subcarrier according to anembodiment of the present invention described above whereas the secondsub-carrier (not shown) in both cases are conventional binaryrectangular waveforms, that is, conventional subcarriers. It can beappreciated that there are regions 1114 of overlap between the twoBOC(2,2) subcarriers 1104 and 1110 and their resulting products, thatis, 16 MHz subcarrier modulated BOC(2,2) signals 1106 and 1112. In theregions of overlap 1114, the waveforms have the same amplitude profile.

An advantage of the embodiments of the signals shown in FIG. 11 is thatthe I channel or component has been produced or represents DBOC8modulation or signal whereas the Q channel has been produced using orrepresents BOC8 modulation. However, this arrangement still preserves orprovides a substantially constant envelope carrier signal to be emittedfrom the satellite.

Embodiments of the present invention have been described with referenceto the subcarrier signals being periodic. However, embodiments can berealised in which the subcarrier signal comprises a pseudorandom noisesignal. Furthermore, embodiments can be realised in which the shape ofthe subcarrier takes a form other than a stepped, that is, a multilevelwave or quantised approximation of a sinusoidal waveform. For example,multilevel-pulsed waveforms, multilevel-periodic waveforms ormultilevel-aperiodic waveforms, could be used such as the signal shownin FIG. 12 according to the influence one skilled in the art wishes theresulting modulation to have on the power spectrum of the transmittedsignal and/or any appropriate measure of interference such as, forexample, SSC or self-SSCs.

Referring to FIG. 13, there is shown a subcarrier waveform 1300according to a further embodiment of the present invention together withone chip 1302 of a code or other waveform such as, for example, anothersubcarrier. It can be appreciated that the subcarrier comprises a firstportion of a BOC(5,1) waveform, in the 100 ns sections, combined withportions of a BOC(1,1) waveform, in the 400 ns portions, to produce anoverall subcarrier. It will be appreciated that the spectra of theBOC(5,1) waveform will have a peak at 5*1.023MHz and the BOC(1,1)waveform will have a peak at 1*1.023MHz. Therefore, one skilled in theart appreciated that selectively combining the BOC subcarriers allowsone skilled in the art to position or relocate the peaks of the overallsubcarrier. Again, it can be appreciated that the subcarrier used, forexample, to modulate the ranging codes is derived from more than onesubcarrier. Although the signal described in relation to FIG. 13 hasbeen derived from BOC(5,1) and BOC(1,1) subcarriers, embodiments can berealised in which other combinations of BOC subcarriers are used, Ineffect, the BOC(5,1) and BOC(1,1) signal have been multiplexed orselectively combined to produce an overall subcarrier signal. It will beappreciated that other sequences for the subcarriers can be realisedaccording to a desired effect upon the power spectrum of a transmittedsignal. For example, a subcarrier can be realised using a pseudorandomsequence as sub-carrier instead of the stepped modulations. The use ofadditional sequences to that of the main spreading code has hithertobeen limited to use as a tiered code, which changes state after everycomplete code repetition interval. The GPS L5 codes are constructed inthis, manner using Neumann Hoffman sequences of length 10 or 20 toextend a 1 ms code (of 10230 chips or elements) to 10 ms or 20 ms. Theuse of a subcode chip interval has not previously been considered. Acomplete sequence (a sub-sequence) has a duration of one code chip, orat most a plurality, of code chips. It fulfills a similar role to thesub-carrier modulation as previously described in that it controls thespectrum of the emissions. One feature of such a sub-sequence is thatsuch sequences may be chosen to be common amongst a satelliteconstellation or a sub-set of the constellation. One such subset mightbe a group of ground transmitters providing a local element oraugmentation to the space segment of the system. For example, subcarrieramplitudes can be realised that have the sequence −+++++−−−−+ in 10subchip intervals or other sequence of +1's and −1's per ranging codechip or other subcarrier chip according to the desired effect onspectrum of the resulting signal. Examples such as the 7 subchipinterval sequences would include ++−−−−−, +++−−−−, +−+−+−−, and can bechosen to provide similar control over the emitted spectrum.

Referring to FIG. 14, there is shown, schematically, a transmitter 1400according to an embodiment of the present invention. The transmitter1400 comprises means 1402, that is, a generator, for generating orselecting the ranging codes for transmission. It will be appreciated bythose skilled in the art that such ranging code may be generated by, forexample, shift register implementations. It can be appreciated that theranging code selection and/or generation means 1402 is illustrated asproducing g_(i)(t) and p_(i)(t). These codes are fed to respectivemixers 1404 and 1406. The mixers 1404 and 1406 are arranged to combinethe ranging codes with subcarriers according to embodiments of thepresent invention. Respective subcarrier generators 1408 and 1410generate the subcarriers. Optionally, a data signal, d_(i)(t), is alsopreferably mixed with the ranging codes and subcarriers. The duration ofone bit of the data signal is normally an integer multiple of the coderepetition interval. For example, in GPS CA code, it is 20 times the 1ms code repetition interval, that is, the data rate is 50 bps. The mixedsignals 1412 and 1414 are fed to a further pair of mixers 1416 and 1418,where they are mixed with in-phase and quadrature phase signals producedvia an oscillator and phase shifter assembly 1420. The further mixedsignals 1422 and 1424 are combined, via a combiner 1426, and output forsubsequent up conversion by an appropriate up converter 1428. The outputfrom the up converter 1428 is fed to a high-power amplifier 1430 andthen filtered by an appropriate filter 1433 for subsequent transmissionby, for example, a satellite or other device arranged to emit ortransmit the ranging codes.

Referring to FIG. 15, there is shown a schematic representation of amodulation system 1500 according to an embodiment. The system 1500comprises a ranging code generator 1502 for producing a ranging code.The ranging code is fed to a first lookup table 1504 comprising phasestates and a second lookup table 1506 comprising amplitude states. Theoutput of the phase state lookup table 1504 is used to drive a phasemodulator 1508, which, in turn, produces a voltage signal to control thephase of a voltage controlled oscillator 1510. The output of theoscillator 1510 is combined, via, a combiner 1512 such as, for example,a gain controlled amplifier or multiplier, with a signal output from theamplitude state table 1506 to produce a subcarrier having theappropriate characteristics.

Although the above embodiments have been described with reference tomaintaining a substantially constant signal envelope, embodiments arenot limited thereto. Embodiments can be realised in which variablemodulus signal envelopes are used. It will be appreciated that theconstraints described above, which are aimed at preserving unitarymagnitude of (I²+Q²)^(1/2), need not necessary apply.

The above embodiments have been described with reference to the I and Qchannels having the same chipping rates. However, embodiments are notlimited to such arrangements. Embodiments can be realised in whichdifferent chipping rates are used.

Although embodiments of the present invention have been described withreference to the L1 and L2 frequencies, embodiments are not limited tosuch arrangements. Embodiments can be realised in which otherfrequencies or frequency bands can be used according to the requirementsof the system using the invention. For example, the lower L band (i.e.E5a and E5b), the middle (i.e. E6) and upper L-band (i.e. E2-L1-E1) canalso benefit from embodiments of the present invention. It will beappreciated that such embodiments may use signals having at least threecomponents rather than the two components described above.

Furthermore, embodiments of the present invention have been describedwith reference to standard BOC. However, one skilled in the artappreciated that embodiments can also be realised using Alternative BOC.

Furthermore, it will be appreciated that embodiments can be realised inwhich the number of half cycles of a subcarrier per chip of a code canbe at least one of odd, even, an integer multiple or a non-integermultiple of the chip, that is, there is a rational number relationshipbetween the number of subcarrier half cycles and the chip duration.

Embodiments of the present invention described above have focused on thetransmission side of the invention, that is, upon the generation,modulation and transmission of ranging codes combined with a subcarrieror subcarriers. However, one skilled in the art appreciated that aconverse system and method are required to receive and process thesignals. Once one skilled in the art has designed a system fortransmitting such signals, designing an appropriate receiver is merelythe converse of the transmit operations. Therefore, embodiments of thepresent invention also relate to a receiver for processing signals suchas those described above.

The reader's attention is directed to all papers and, documents that arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A method of generating a subcarrier modulation signal for modulatinga further signal, the method comprising the step of multiplexing orselectively combining portions of first and second subcarriers toproduce the subcarrier modulation signal. 2-97. (canceled)
 98. A methodas claimed in claim 1 wherein said multiplexing or selectively combiningportions of first and second subcarriers to produce the subcarriermodulation signal comprises combining a portion of the first subcarrier,spanning a respective time interval, and a portion of the secondsubcarrier, spanning a respective time interval.
 99. A method ofgenerating a subcarrier modulation signal for modulating a furthersignal, the method comprising the step of selectively combining portionsof first and second subcarriers to influence thereby the location ofspectral peaks associated with said first and second subcarriers withinthe spectrum of the subcarrier modulation signal.
 100. A subcarriermodulation signal for modulating a further signal, the subcarriermodulation signal comprising multiplexed or selectively combinedportions of first and second subcarriers to produce the subcarriermodulation signal.
 101. A subcarrier modulation signal as claimed inclaim 100 wherein the portion of the first subcarrier comprises aportion of the first subcarrier, spanning a respective time interval,and the portion of the second subcarrier comprises a portion of thesubcarrier, spanning a respective time interval.
 102. A subcarriermodulation signal comprising multiplexed or selectively combinedportions of first and second subcarriers.
 103. A subcarrier modulationsignal comprising selectively combined portions of first and secondsubcarriers arranged to influence thereby the location of spectral peaksassociated with said first and second subcarriers within the spectrum ofthe subcarrier modulation signal. 104-165. (canceled)